Project Summary Human p300 is a transcriptional co-activator and a major acetyltransferase that acetylates histones and other proteins, facilitating gene transcription. P300 is required for a wide array of cellular programs, including cell growth, differentiation, apoptosis, and DNA repair, and is implicated in human diseases, cancer and genetic disorders in particular. P300 and its paralog CBP are among the most frequently mutated genes in human malignancies. p300 contains a unique assembly of the catalytic histone acetyltransferase (HAT) domain closely linked to the ZZ domain, the biological role of which remains unclear. We have identified the ZZ domain of p300 (ZZp300) as a novel epigenetic reader that recognizes histone H3 tail. Our findings suggest that this recognition mediates catalytic and chromatin binding activities of p300. The molecular mechanisms and functional significance of these novel p300 activities are unknown and are the focus of the proposed studies. We hypothesize that the interaction of ZZp300 with H3 promotes selective acetylation of the histone H3K27 and H3K18 sites by p300. Furthermore, we propose that ZZp300 cooperates with bromodomain (BDp300) in the recruitment and stabilization of p300 at chromatin. We will employ a powerful combination of in vitro and in vivo approaches to establish the molecular and structural basis, mechanistic insights, and biological significance of crosstalk between histone readers in p300. This research is of fundamental importance to our understanding of the physiological and pathogenic activities of p300. These studies will also shed light on the role of the novel epigenetic reader, ZZp300, allowing us to build a model of signaling by this major acetyltransferase, and will lead to a better understanding of human diseases associated with aberrant p300 activity, including cancer and genetic disorders.